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With the generic term optical aberration It covers a series of defects that affect optical instruments with lenses and mirrors.
In the chromatic aberration the different colors (wavelengths) that make up the light, when passing through a lens they are deflected in different ways and give rise to the formation of an image contoured by the colors of the rainbow. In a biconvex lens, for example, violet rays converge towards the focus before the red ones. The defect is eliminated by resorting to an achromatic system composed, in its simplest form, of two lenses, one called "flint" and the other "crown", whose refractive index is different. Mirrors lack chromatic aberration.
The spherical aberration, on the other hand, affects both the lenses and the mirrors and is due to the fact that the peripheral parts of a lens or a mirror, make the light rays converge towards a slightly displaced focus with respect to that of the central parts, giving rise to an unfocused image.
Astigmatism is a defect of some optical systems consisting of the inability to drive light rays from different planes to a common focus, for example, the horizontal and vertical planes. If a star with an astigmatic defect eyeglass is observed, an ellipsoidal image will be observed instead of a pointed image. To correct astigmatism, the use of toric lenses (bull revolution) is generally used, which have a spherical face and an toric face. However, at some point cylindrical and spherical cylindrical lenses were used.
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